New types of electrical energy storage devices are needed to power electronic devices, electric vehicles and to smooth peak power demands on electric utilities. Some promising devices are based on the use of a solid electrolyte separator such as ceramic, beta alumina solid electrolyte (BASE).
Since the discovery in 1962 that the material .beta."-alumina was a good sodium ion conductor, several studies were made on its use as solid electrolyte separator in various battery systems with liquid sodium as anode. The interest for many years has been focused on sodium-sulfur battery, which has many attractive features such as high density, high rate discharge capability permitted by a good (comparable to aqueous electrolytes) ionic conductivity of .beta."-alumina solid electrolyte (BASE) at high temperatures and long cycle life and negligible self discharge. However, there are certain difficulties associated with the use of sodium-sulfur batteries. In particular, due to the highly corrosive nature of sulfide melts, material selection for the current collector in the positive electrode is very critical and limited to a few possible choices. Also, there is a likelihood of BASE degrading in polysulfide melts. Further, the inherent violent reaction between liquid sodium and liquid sulfur demand a rather sophisticated design of the battery to circumvent the safety problem in the event of failure of the solid electrolyte ceramic.
A new class of high temperature, sodium rechargeable batteries based on transition metal chlorides as positive electrodes have emerged in the last decade. These systems are similar to the sodium-sulfur batteries in terms of anode half cell and the (high) energy densities. In addition, the use of solid metal chloride cathodes in basic chloraluminate melts results in several significant advantages, including lower operating temperatures, improved safety and high reliability. Excellent performance characteristics have been demonstrated with both Na/FeCl.sub.2 and Na/NiCl.sub.2 systems in small and large cells as well as in batteries.
Sodium/transition metal chloride cells have lower power densities than sodium-sulfur cells. This has been attributed to higher polarization losses at the cathode. There have been proposals to reduce polarization at the cathode by modification of the .beta."alumina solid geometry to flat plate or multiple tube configurations. However, such configurations are not compatible with the present tubular BASE electrode design and would necessitate a major change in the methods of fabricating solid electrolytes for sodium/transition metal chloride cells.
Furthermore, an optimization study of Na/NiCl.sub.2 batteries for space applications indicated that the optimum thickness for the cathode is 4 mm; electrodes thicker than 4 mm from the current collector have a reportedly high diffusional polarization and low utilization efficiencies. It may be difficult to achieve reasonable energy densities with Na/NiCl.sub.2 cells if the cathode is constrained to these dimensions.
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